Tonight, millions of americans are bracing for impact, as hurricane sandy rapidly approaches, threatening to morph into a rare superstorm as it barrels up the east coast. Abc's ginger zee of our extreme weather team is on the ground in florida with the latest. Reporter: As hurricane sandy rumbles through the atlantic, just a few hundred miles from the east coast, its path of destruction has already killed at least 30 people across the caribbean. From space, you can clearly see the size of this thing. It has tropical storm force winds extending almost 300 miles. It's shield of clouds stretching almost 2,000 miles. The storm is far enough away that florida will escape the brunt of the danger, but what we can't escape is beach erosion. The search punding up and over. This should not be happening right here. The beach usually starts about 50 yards out. Through the weekend, sandy will snake around the coast, lashing the carolinas with its western-most tentacles before making its dirty turn directly toward the u.S. Somewhere from the mid-atlantic to new england. When you factor in the unusual track that it's going to be taking, that is coming in from the east, across the jersey coast, it's a once in a lifetime storm. Arer the Reporter: The so-called once in a lifetime storm will only get worse as it moves towards land. Monday, it will collide with energy that will pull in some cold air from the northwest. Now, adding to that perfect storm scenario, there's what we call a blocking pattern that shoves energy from the north back at the storm and back toward the united states. Here's what all of that means to you. The storm will carry hurricane-force winds, up to a foot of rain and the potential storm surge that could top out at ten feet. Extreme conditions could begin as early as sunday and last for days, drenching the most populated part of the country. This storm, that is going to be impacting the mid-atlantic and parts of the northeast monday into tuesday is going to be destructive, historic and, unfortunately, life threatening. Reporter: A state of emergency has already been declared in maryland, d.C., pennsylvania, new york and virginia. The biggest fear? The storm surge. In places like atlantic city, it could get even worse because of a full moon and higher tides. People are batoning down the hatches, preparing for the worst, up and down the eastern seaboard. In norfolk, the navy ordered 23 ships to leave port immedi on the jersey shore, people are stocking up on essentials, including those generators. And in new york city, major bloomberg warned he may have to shut down the subway because it miflood. There is a possibility that the mta would have to stop serving, remove their buses and trains. Reporter: The storm is also wreaking havoc on the presidential race, with the candidates already beginning to cancel events. It has all the makings of a nightmare before halloween. For "nightline," I'm ginger zee in cocoa beach, florida.

This transcript has been automatically generated and may not be 100% accurate.

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